Help me plz. i really need advice . im asking sincerly for help.?

Me and my now x had been together for 4 yrs. We broke up for about half a year and we recently got bk. At first everything was good and gravy,but this last week shes really been acting different,less calls,no urge to c me and i started to become suspicious of her. Ladt night after dinner and a couple drinks in us i decided to to ask her for her phone, she became defensive and asked me let me see yours and i agreed so she starts showin me her phone. On her recent calls she had call after call from her girl kuzn but it seemed weird so i wanted to see for myself thats when she flipped, she locked her phone and said i wasnt going through anything. If it was me doing this to her she would of flipped out and all hell would of broke loose. She would always let me go through it before our last break up and i wouldnt be constanly checking it kuz i was secure of myself but she would constanly go through mine, she didnt bother to go through this time kuz she knows im legit. But keep in mind that she did show me her phone but the way she showed me was so obvious she was hiding sumthin. We have always made it clear when we felt insecure about one another but this time with tears in my eyes tellin i was good to her that would she do that to me and her response was well its my loss then. Its so hurtful bkuz i have always been so caring for her and have been there for her. Shes a very attractive girl but my friends tell me i could do way better but shes managed to bring my self esteem way down. Wut do i do?? Wut should i think?? Help plz.